The invention relates to milling machines which can be used to machine both eccentric end faces and circumferential faces, e.g. peripheral faces.
A typical workpiece having such faces is a crankshaft, for which reason the following text always refers to a crankshaft, without limiting the possible workpieces to this alone.
For crankshafts, metal-removing machining is known both with external-milling machines and with internal-milling machines, that is to say by means of a milling cutter which annularly surrounds the crankshaft and has inwardly directed teeth. In this case, the axis of rotation of the milling cutter lies parallel to the longitudinal axis of the crankshaft.
In this case, the crankshaft is held at its ends, that is to say on one sides at its end flange and on the other side at its end journal, centrically, that is to say on the centre axis of its centre bearings, in chucks on both sides.
In a known solution, the crankshaft does not move during the machining, and thus the chucks are not driven by a spindle head. For machining of the crankpin journals, the annular internal-milling cutter rotates, on the one hand, about its own centre point in order to generate the cutting speed and, on the other hand, on an orbit about the centre of the crankpin journal to be machined, in order to mill the peripheral face thereof. Web end faces and web circumferential surfaces can also be machined in this manner, as long as the radius of curvature is smaller than the radius of the circuit of the cutting edges of the internal-milling cutter. With the crankshaft stationary, the internal-milling cutter can be displaced in a defined manner in the X- and Y-directions.
The mounting which annularly surrounds the internal-milling cutter is extremely stable, but has a relatively wide extent in the Z-direction, for which reason, for short crankshafts, the simultaneous deployment of two internal-milling cutters axially spaced apart on the same crankshaft can be problematical.
It is also known in that solution to rotate the crankshaft slowly during the machining, that is to say to be able to drive at least one of the chucks in a defined manner by means of a spindle head and to set its rotational position. This realisation of the C-axis for the workpiece makes it possible to dispense with the movement of the tool slide rest in the Y-direction, so that therefore only the tool slide rest for the internal-milling cutter merely comprises a lower slide for movement in the Z-direction and an upper slide for movement in the X-direction.
Furthermore, external-milling machines are known, in which the milling units--in addition to the displaceability in the Z-direction--were displaceable in a defined manner in the X-direction, and the chucks for the crankshaft were held in one or two spindle heads. Realisation of the C-axis on the workpiece meant that the external-milling cutter was guided on in a defined manner in the X-direction during the machining of eccentric surfaces. However, the machining of eccentric surfaces did not entail two or more external-milling units, operating independently of one another, being deployed on the same crankshaft. This was only possible when machining the eccentric surfaces, e.g. the centre-bearing journals.
In the case of the known milling machines, work is carried out with a conventional, negative cutting-edge geometry and cutting speeds on workpieces made of grey cast iron (GGG60-GGG80) of at most 160 m/min. As a result, very high cutting forces are introduced into the workpiece, for which reason it is also necessary as a rule to support the centre of the workpiece by means of steady rests, etc. A further drawback consisted in the fact that a high proportion of the process heat was introduced both into the workpiece, and thus also into the tool, and only a small proportion was dissipated via the chips.